That is not quite true. If you take the case of the a7n8x deluxe & a7n8x-e deluxe, there are changes between the two boards.The -E comes with an onboard Gigabit NIC & WIFI slot.

Generally, the -E boards are more recent & might have a few tweaks that resolve issues present on the previous versions such as better board layout, etc.

As for the differences between the P4C800 Deluxe & P4C800-E Deluxe, the northbridge heatsink has been placed differently & the presence of the new Intel Southbridge ( ICH5R ) & Intel Gigabit on the CSA GigaLAN instead of the 3Com Gigabit via the PCI bus.

That's not true. P4C stands for Canterwood which is the chipset the board happens to be using. Both of these boards use the Canterwood chipset thus both start with P4C, there's only the extra E at the end of the name which probably stands for "extra features".